Piston



May 13, l193). C, WALKER 1,758,302

` PIsToN Filed May 25, 1928 IZzgJa.

Patented .May is, 1930 ,UNITED STATES PATENT' OFFICE CLINTON L. WALKER, F PIEDHONT, CLIFOBNIA PIsroN f z `Application led lIllia? 23,

` This invention relates to pistons and particularly that-'typelemployed in internal combustion engines.

In the operation of internal combustion engines of the high speed type, it is desirable to make pistons particularly 'of the trunk type, as light as possible. The use of aluminum or alloys thereof in piston construction has been extensivelv attempted and solves theproblem of weight reduction.. Aluminum,

however, has a greater coeiiicient of expansion than cast iron and asa r "ult considerable clearance must be allowed between the piston` iron or steel; to provide a piston which will' expand or contract in diameter in harmony with the expansion or contractionof the bore of the cylinder :and further to produce a piston which. will have a longer life and offer better friction conditions.

One form which my inventionmay assume is exemplified in v the following description and illustrated in the accompanying drawings', in which:

Fig. 1 is a side elevation of the piston, showing the application of the cast iron shell.

Fig. 2 is a central vertical section of the piston. K

' Fig. 3 is a cross section on line IIIe-III, 40 Figure 1.

Referring tothe drawings in detail. and particularly to Figures 1 and 2, A indicates the head of the piston, B the ring portion` and C the skirt. The ring portion is provided with the usual grooves 2 for the reception of peratures; and further to produce a pisten and the shell. This is accomplished by pro- 1928. -Serial No. 2719,953. A

the piston rings, and the interior of the piston is provided with the usualbosses 3 for the reception of thie wrist pin. 4The skirt portion'is in this instance reduced in diameter substantially from end to end for the purpose ,of receiving al cast iron shell generally indicated at E, and large grooves 7 and 8 are formed at the upper and lower ends of the re' duced portion, these grooves being provided with inclined seats as indicated at 9 and 10. 55

. The shell proper may be constructed of cast iron or steel and it is made in two or more segments, as indicated at E and F, the line of division between the segments being shown at 4:. The upper and lower. ends of the shell 60 sections E and F are provided with inwardly turned annularly disposed flanges 11 and 11,V

which engage the seat-'members 9 and 10,

and the exterior surface of the' shell sections are grooved as indicated at 12, tore- I ceivebabhitt rings as the babbittV 1s poured into the grooves and is lafterward machined and finished to the proper diameter.

The purpose of this invention is to con- -Struct 4a composite piston consisting of an aluminum body. and an exterior shell, which will maintain a constant degree of contact with the cylinder walls particularly along the sldrt portion,`when subjected to varying tem-y which will oier better friction conditions. The co-ecient of expansion of aluminum compared with cast iron is 1.9 to 1; that is, aluminum expands nearly twice as much as iron orvsteel. Hence, if the piston is heated, the gap indicated at L opens ,up more than the length of the cast iron shell and it is therefore necessary to compensate for this unequality in expansion between the piston skirt viding the i 1clined annularseats 8 and 9 and the inturned angularly disposed ianges 11 and 11a; that is, the diameter o f the skirt portion always increases when the piston is heated and this increase in diametercom.,

pensates for the increase in the length of the gap indicated at L. This compensation permits the shell or shoe sections EY and F to maintain a constant contact with the inner walls of the cylinder providing the angle of the seats 9 and 10 iscorrect. This angle is found from the equation that the tangen of the angle L (EpTp ESTs) I D (EpTp EcTc) where L is the length of the shell and Es and T8 its coeiiicient of expansion and temperature change, Ep and Tp the coeicient of expansion and temperature change of the body or skirt -oi'tion of the piston and D its diamd) E0 and Tc the coetlicient of expan-` eter, an sion and temperature change of the cylinder wall. The temperature changes in the piston skirt, in the shell and in the cylinder wall would have to be observed under all variations of motor conditions of operation before applying the formulae, the greatest change being just after starting a cold motor.

In actual practice, when designing pistons of this character for a given size or type of motor, the angle O is first determined by calculation, by using the above formula.

For the sake of description, it will be as-- sumed thatthe calculated angle is 38 degrees. In that case a number of pistons will be made, one having an angle of 39 degrees, another 38 degrees, another of 37 degrees, another of 36 degrees, etc. These several pistons will be placed one by one, in the motor and operated under testing conditions. While testing these pistons it may be found that the 39 degree angle has a tendency to seize the moment the motor heats up; the 38 degree angle may also have a tendency to seize; the 37 degree angle may operate satisfactorily under `all temperature conditions and loads, and the exact angle suitable for the particular type of motA tor and piston diameter is thus not only c al.-

culated, but practically determined.

The following formula may thenv be developed from the calculated and practical test:

in which K is a variable, determined by experiments, for instance in this case .81 L represents the length of the shell and D the diameter ofthe piston.

n should be realized am as the diameter pe of the piston increases, new temperature conditicns are introduced which may change the variable K, similarly the variable may be changed due to a change in the coeilicient of expansion of the particular aluminum alloy employed. It is therefore obvious, whenever a new motor is to be fitted with pistons of this character, that no iron clad rule or formula can be followed, as the angle must first be extra stock between the ring theoretically calculated, and finally deter-l mined by experimentation. For a given motor, with a given or known alloy, the variable will become a constant.

In the present case, it will be observed that the trunk portion of the piston will heat up more rapidly than the water jacketed cylin- "der and the piston shell which is in contact with it. The change would undoubtedly be nearly the same in the shell segments and the cylinder wall as the shell segments have contact with the skirt portion of the pistons only 'at the ends and wlth the cylinder walls at the rubbing portions of the shell segments. When the angle indicated at O is correctly determined the exterior diameter lof the shell sections E and F may be turned to the exact bore of the cylinder with no allowance for expansion as the exterior diameter will remain in constant running contact with the cylinder wall, regardless of contraction or expansion of the 'aluminum piston skirt. This will produce a quiet running motor. It will reduce piston vibration and weight, the piston will wear longer, and no slap will be encountered when the piston is cold. The

babbitt rings indicated at 13 are also important as they maintain the cast iron or steel shell out of actual contact with the cylinder wall and as such' will offer better friction conditions, and when wear does occur the babbitt can be renewed without discarding the east iron shell or the piston propen It will be observedthat the shell is in con-l by merely returning the grooves and by rereaming the wrist pin holes and inserting oversized wrist pins. The babbitt rings in the shell can be renewed from time'to time,

pistons Awith wear.

The babbitt wea friction conditions an ted that the cylinder 'will wear far less than with the conventional type of piston.

thus making it unnmto the surface oiers'better hence it is to be ex" This longer wearing life of the cylinder will also be augmented by reason pf the elimination o f piston slap w en the pistons are cold.

WhileV certain features :of the present invention are more or less specifically illustrated, I wish .it understood that various changes in form and proportion may be resorted to within the scope of the appended claims. I similarly wish it understood that l gular seats at the yupper and lower ends of register with and support inturned beveled the materials and finish of the several parts employed may be such as the experience and judgment of the manufacturer may dictate or various uses may demand.

Having thus described my invention, what .I claim and desire to secure by Letters Patent 1s:

1. A piston comprising a headand a sln'rt portion, said piston being constructed of an aluminum alloy and said skirt portion having an elongated recess formed therein with anthe recess, and a metallic shell adapted to be received in the recess and having its major surface out of Contact with the skirt, said shell having angular faces formed at its opposite ends conforming to the angular seats in the recess and the angle employed being that angle Whose tangent is equal to the difference between the longitudinal e'xpansion of the piston portion between the angular seats and the longitudinal expansion of the shell member divided by the diametrical expansion of the skirt member minus the diametrical expansion of a cylinder bore when the piston and cylinder .are subjected to operating temperatures.

2. A piston comprising a head, a skirt and a shell member, said skirt member having a longitudinal recess with beveledv ends to ends on the shell member, the angle of the bevels of the skirt and the shell members at' each end to a central diametrical transverse plane being such that the' trigonometrical tangent of thisA angle is equal to the longitudinal expansion of the recess in the skirt A member minus the longitudinal expansion of the shell member, divided by the diametrical expansion of the skirt member min-us the diametrical expansion of the cylinder when the piston is subjected to operating conditions, said shell having its major surface out of contact with the skirt.

CLINTON L. WALKER. 

